A more in-depth look at the Republicans-for-Hillary phenomenon ...
Now that their nomination race is over, Republicans will feel freed up to vote in Democrat contests, meaning that the summons to cross party lines by Mr Limbaugh, who has a weekly audience of at least 14 million, could play a significant part in deciding the next US president.
Exit polls in Texas and Ohio show that nine per cent of voters in both were self-declared Republicans.
Last week Mrs Clinton's share of that vote more than doubled. In nine previous open primaries, no more than six per cent of voters were Republicans, and Mr Obama won overwhelmingly each time, with one exception.
Laura Kreissl, an accounting professor who was volunteering at precinct 307 in Canyon, a town 20 miles south of Amarillo, said she was "stunned" when the first person she checked in said "Rush Limbaugh sent me".
"He said: 'I am voting for Hillary Clinton but I want to see the Democrats implode'. As I sat there for 12 hours you hear people rattling on, and a great many mentioned Rush Limbaugh," she said.
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